Conventionally, among scanning line interpolating devices for conducting a scanning line interpolation process that converts an interlaced scanned video signal into a progressively scanned video signal and the like, there have been scanning line interpolating devices, which have a function of determining the pixel value of an interpolation-target pixel based on correlation of at least two pixels located in a diagonal direction from the interpolation-target pixel (hereinafter, referred to as diagonal interpolation function). With the scanning line interpolating devices having a diagonal interpolation function, a diagonal edge can be properly reproduced.
Among the scanning line interpolating devices having the diagonal interpolation function as described above, in order to suppress image quality degradation which occurs when a false operation of the diagonal interpolation process happens, there are those that compensate a pixel value such that a pixel value falls within an interval between the pixel values of the adjacent pixels located above and below, when the pixel value determined by the diagonal interpolation process is beyond the interval between the pixel values of the adjacent pixels located above and below (refer patent document 1 as an example).    Patent document 1: Japanese Laid-Open Patent Publication No. 2002-185934
FIG. 10 is a block diagram of a configuration of a conventional scanning line interpolating device that includes a function of compensating a pixel value determined by the diagonal interpolation process as described above.
In FIG. 10, an interpolation processing section 10 determines a pixel value of an interpolation-target pixel based on correlation of two pixels located in a diagonal direction from the interpolation-target pixel, based on a video signal of a scanning line just above the interpolation-target pixel (hereinafter, referred to as video signal on the upper line) and a video signal of a scanning line just below the interpolation-target pixel (hereinafter, referred to as video signal on the lower line).
An interpolation value limiting section 12 judges whether or not the pixel value determined by the interpolation processing section 10 is beyond the interval between the pixel values of the adjacent pixels located above and below the interpolation-target pixel, based on: the pixel value determined by the interpolation processing section 10, and the video signal on the upper line and the video signal on the lower line. When the pixel value determined by the interpolation processing section 10 is beyond the interval between the pixel values of the adjacent pixels located above and below the interpolation-target pixel, the interpolation value limiting section 12 compensates the pixel value determined by the interpolation processing section 10 such that the pixel value determined by the interpolation processing section 10 falls within the interval between the pixel values of the adjacent pixels above and below, and outputs the compensated pixel value as a compensated video signal. When the pixel value determined by the interpolation processing section 10 is not beyond the interval between the pixel values of the adjacent pixels located above and below the interpolation-target pixel, the interpolation value limiting section 12 outputs the pixel value determined by the interpolation processing section 10 as a compensated video signal without compensation.
FIG. 11 shows, with regard to a video signal having a diagonal edge (color boundary line in a diagonal direction), a relation between video signals on the upper line and on the lower line, and a desired interpolated video signal. Here, when a pixel value beyond the interval between the pixel values of the adjacent pixels located above and below the interpolation-target pixel (e.g. “incorrect interpolation value” in FIG. 11) is outputted from the interpolation processing section 10 due to a false operation of the diagonal interpolation process, this pixel value is compensated by the interpolation value limiting section 12 so as to fall within the interval between the pixel values of the adjacent pixels above and below the interpolation-target pixel as shown by an arrow in FIG. 11 (e.g. compensated to an average of pixel values of the adjacent pixels above and below the interpolation-target pixel). In this way, since an incorrect interpolation value is compensated so as to approach a desired interpolation value, it is possible to suppress image quality degradation that occurs when a false operation of the diagonal interpolation process happens.